Role of Hox genes in regulating digit patterning.

The distal part of the tetrapod limb, the autopod, is characterized by the presence of digits. The digits display a wide diversity of shapes and number reflecting selection pressure for functional adaptation. Despite extensive study, the different aspects of digit patterning, as well as the factors and mechanisms involved are not completely understood. Here, we review the evidence implicating Hox proteins in digit patterning and the interaction between Hox genes and the Sonic hedgehog/Gli3 pathway, the other major regulator of digit number and identity. Currently, it is well accepted that a self-organizing Turing-type mechanism underlies digit patterning, this being understood as the establishment of an iterative arrangement of digit/interdigit in the hand plate. We also discuss the involvement of 5' Hox genes in regulating digit spacing in the digital plate and therefore the number of digits formed in this self-organizing system.